Demantra Forecast Tree Question

Hello All,

I want to understand the logic of lowest item level & lowest location level as well as Highest Fictive level during configuring Forecast tree.

For ex, Forecast tree set up is like below::

1)Lowest item level - Lowest location level

2)SKU - Site

3)Highest Fictive Level - Highest fictive level.

So in above forecast tree set up, level1 & level3 are the default set up which would not consider during engine run.This is just a kind of set up which needs to follow. Both level1 & level3 are fictive levels which is not useful  This is my understanding.

Is there any significant meaning of level1 & level3.